feat: add templates for python samples projects

[busunkim96]

These templates will be used for templates in python-docs-samples
and in Python client libraries.

The README generation code is a modified
version of https://github.com/GoogleCloudPlatform/python-docs-samples/tree/master/scripts/readme-gen.

For new samples, what are the steps for generating a new noxfile? Is there a way for users to manually generate it to ensure they get an updated version?

To finish setup for a new set of samples in foo/:

• client libraries: Go to the root of the repository, edit the synth.py, and run synthtool.

    import synthtool as s
   import synthtool.gcp as gcp
  +from synthtool.languages import python
   import logging
  
   logging.basicConfig(level=logging.DEBUG)
  @@ -71,7 +72,13 @@ if count != 1:
   # ----------------------------------------------------------------------------
   # Add templated files
   # ----------------------------------------------------------------------------
  -templated_files = common.py_library(cov_level=70)
  +templated_files = common.py_library(cov_level=70, samples=True)
   s.move(templated_files)
  
  +# ----------------------------------------------------------------------------
  +# Samples templates
  +# ----------------------------------------------------------------------------
  +
  +python.py_samples()
  +
   s.shell.run(["nox", "-s", "blacken"], hide_output=False)
  

• python-docs-samples: Copy synth.py from the root of the repo and run synthtool.

  $ cp synth.py foo/synth.py
  $ cd foo
  $ synthtool

  (You can also run synthtool from the root of the repository, but it will attempt to regenerate all the noxfiles and readmes.)

[kurtisvg]

For new samples, what are the steps for generating a new noxfile? Is there a way for users to manually generate it to ensure they get an updated version?

[leahecole]

For new samples, what are the steps for generating a new noxfile? Is there a way for users to manually generate it to ensure they get an updated version?

Just a comment that whatever we decide the steps are for new samples, they need to be added to the authoring guide and somehow communicated outward to our lovely contributors

[leahecole]

Will we need to add a .yml to every subdirectory? or only the ones whose noxfiles are different than the generic one?

[busunkim96]

@SurferJeffAtGoogle Is it alright to use templates directly or will it break something related to the synth metadata tracking?

[busunkim96]

@leahecole Every single one, but I tweaked the logic so that synthtool will add a default noxfile.py.yml if one isn't found.

On an unrelated note this ended up being more code than I expected so I will add some tests.